My View on Choosing Friends

As the saying goes: A man is known by his friend. This tells us to be slow and careful in choosing a friend. Now I want share some views on choosing friends with you.
I think the most important quality of a friend is loyalty. The story of Taogong teaching his son about choosing friends is widely known in China. Taogong pretended his son killed a person and then asked friends for help. His son’s so-called friends all avoided involving themselves in trouble, but Taogong’s friend tried his best to help his son. From this story, we learn that a true friend never leaves you when you are frustrated or when you lose.
Optimistism is also very important. A friend is a mirror by which you can see the second self. If your friends are optimistic and positive, then he or she may affect you to be like themselves. You will learn to smile and with the smile, face the difficulty in life.
It is nature that different people have different principles about choosing friends, although measures are not the same, but I believe everyone wants have a true friend.

PS: Thank you in advance ,please help me correct chinglish : )

[Quite good. Remember to put a space after periods and commas, not before them.]